The present disclosure relates generally to a control method for a system including a plurality of elements (or plurality of subsystems of elements), each performing a process using local process control to maintain operation of the process within a latitude around a setpoint and having an output characteristic that contributes to an attribute of an overall output quality specification of the system. More particularly, the present disclosure relates to a control method that dynamically re-sets the setpoint and/or re-allocates the latitude of at least two of the plurality of elements to compensate for an element operating near or beyond its latitude edge so as to preclude degradation of an output characteristic of one of the elements and maintain the attribute within the overall output quality specification of the system during operation.
The control method may have utility in many applications. In one application, the control method may have utility in a printing system in which two or more process elements sequentially perform different image forming processes having different output characteristics that contribute to one or more attributes of an overall output image quality specification of the system. Examples of such printing systems include an electro-photographic or xerographic system, an ink jet printing system, a thermal printing system, and the like. In another application, the control method may have utility in a printing system in which two or more like or similar print engines (or marking engines), or two or more different print engines having the same or similar output attribute(s), are used in parallel to generate high page volume output, satisfying an overall output image quality specification. An example of such a printing system is a document processing system with multiple marking engines.
The present disclosure also relates to a system using the control method, and computer readable recording media containing the control method.
Systems including a plurality of elements (or plurality of subsystems of elements) each performing a process using local process control to maintain operation of the process within a latitude around a setpoint and having an output characteristic that contributes to an attribute of an overall output quality specification are known. For example, in a known xerographic or electro-photographic printing system, the plurality of process elements may include: a photoreceptor, a charging device, an exposing device, a developing device, a transfer device, a fixing device, and the like.
The process elements perform respective system processes. For example, in a known xerographic or electro-photographic printing system, the image forming processes may include: rotating/moving the photoreceptor; charging a surface of the photoreceptor; exposing the charged photoreceptor to light to form a latent image on the photoreceptor; applying toner particles to the latent image to develop the latent image on the photoreceptor; transferring the developed image from the photoreceptor onto a recording media; fixing the toner image on the recording media; and the like.
Each process element uses local process control (e.g., a simple feedback control loop) to maintain operation of each process within a latitude around a setpoint to obtain a desired output characteristic. For example, in a known xerographic or electro-photographic printing system, the control processes may include: a process of controlling rotation of a photoreceptor drum to achieve a rotation speed=Srot+/−ΔSrot; a process of controlling the amount of charge applied to the surface of the photoreceptor to achieve a charge density on the photoreceptor=Vcd+/−ΔVcd; a process of controlling the intensity of light incident on the photoreceptor to achieve a latent image charge density on the photoreceptor=Vicd+/−ΔVicd; a process of controlling the toner concentration of developer applied to the latent image to achieve a toner density=Dt+/−ΔDt; a process of controlling the amount of transfer charge applied at a transfer region=Vtc+/−ΔVtc; a process of fixing the toner image on the recording media by applying a fixing pressure=Pf+/−ΔPf and a fixing heat=Tf+/−ΔTf; and the like. Each local process control typically uses a feedback control loop to maintain each output characteristic (Xi) within a latitude (predetermined maximum permitted ΔX) of a setpoint (X0). That is, X0+ΔX≧Xi≧X0−ΔX
The output characteristics of the plurality of elements contribute to various attributes of an overall output quality specification of the system. For example, in a known xerographic or electro-photographic printing system, the above-discussed output characteristics contribute to a print image density of an output printed recording media. Other output attributes of a printing system include: graininess, contrast ratio, resolution, sharpness of lines/edges, modulation transfer function, line pairs per millimeter, and the like.
In the above example of a xerographic or electro-photographic printing system, a plurality of different elements are arranged and operated sequentially; however, the control method also may be applied to a system including a plurality of like or similar subsystems that are arranged and operated in parallel. In this regard, “like or similar subsystems” means substantially like devices (e.g., plural like xerographic or electro-photographic devices/systems), or plural devices that may function differently but have a like or similar output (e.g., plural printing systems variously including a xerographic or electro-photographic printing system, an ink jet printing system, a thermal printing system and/or another printing system). In each case, the output attributes must satisfy the same overall system output image quality specifications while maintaining sufficient consistency.
A modular document processing system is an example of a system including a plurality of like or similar subsystems, each of which has it own local process control. In a modular document processing system, the subsystems include a plurality of print engines (or marking engines). In an example modular document processing system using plural like devices, each print engine may be a xerographic or electro-photographic printing system; each print engine includes a plurality of sequentially arranged and operated image forming elements (e.g., a photoreceptor, a charging device, an exposing device, a developing device, a transfer device, a fixing device, and the like, as discussed above in the first application). Each image forming element performs a process using local process control to maintain operation of the process within a latitude around a setpoint to obtain an output characteristic; the output characteristics of the plurality of elements contribute to an attribute (e.g., image print density) of an output quality specification of the subsystem (print engine). The output quality specification of each subsystem in turn contributes to the overall output quality specification of the system.
In this modular document processing system example, each subsystem (print engine) nominally is identical; however, in practice, each of the print engines (and its respective elements) will vary slightly due to a number of internal and external factors, including manufacturing tolerances, environmental variations, age/use, and other factors. Each of the setpoints and latitudes required to achieve the desired attribute of the output quality specification of the print engine subsystems likewise will vary from subsystem to subsystem and element to element for each subsystem. Thus, conventionally each subsystem (e.g., print engine) or element (e.g., photoreceptor) individually has been allocated setpoints and latitudes around those setpoints so as to achieve desired local output characteristics and attributes satisfying the common overall output image quality specification.
In each of these conventional systems, if an element or subsystem of elements (e.g., a print engine), exceeds its allocation of latitude and is unable to self-correct by local process control of the element or subsystem, then the element or subsystem, and possibly the overall system, must either (1) shut down, (2) generate a notification that a service action is required (e.g., generate an alarm), or (3) both. Thus, although such conventional systems and control methods have utility in many applications, they have a drawback in that: (1) if one or more elements or subsystems is continuously operating in a region close to exceeding its allocated latitude, then the system operates inefficiently, and (2) if one or more elements or subsystems attempts to exceed its allocated latitude, then the element(s) or subsystem(s) may fail to operate at all.